In many integrated circuit devices it is necessary to amplify the signal very accurately. Since this operation can be repeated several times in the same integrated circuit it requires that the amplification stage by very small both as to size or layout area and as to dissipated power.
A presently known amplification circuit comprises an operational amplifier having an input resistance and a feedback resistance; the amplification value is given by the ratio of the two resistances.
This solution is certainly very good when used used with discrete components, while it displays some serious drawbacks if accomplished in an integrated circuit. It requires long resistive paths and hence considerable layout space or, to limit said space, the capacity to handle high feedback currents with the resulting design difficulties of the operational amplifier and high power consumption.
Another known solution, which is preferred by integrated circuit designers, calls for replacement of the two resistances with two capacitances and the use of a periodically open and closed reset switch in parallel with the feedback capacitance. The presence of the switch is necessary to avoid integration of the leakage current unavoidably present on the negative input of the operational amplifier and in any case, even in case of negligible leakage losses, upon ignition to remove the loads accumulated on the input nodes during the integrated circuit manufacturing process.
This solution is undoubtedly advantageous from the point of view of simplicity of accomplishment and layout space. At the same time the periodic switching of the switch inevitably involves the loss of a part of the input signal and forces the operational amplifier to undergo large voltage jumps at the output with the resulting problems of slow rate and settling time of the output signal.